A variety of methods exist for forming or fabricating various structures from fiber reinforced plastic (FRP). Articles of FRP may exhibit cost advantages over similar articles manufactured entirely of metal because of a reduced number of pieces required, which may result in reduced labor. The production of an article with fewer parts is due to the wide latitude of complex part configurations permissible with FRP molding. However, on structures incorporating metal parts, secondary bonding operations are necessary for high strength bonds between the FRP and metal and generally restrict molding or laminating directly on the metal part. Although primers have been developed for coating the metal, the resulting bond with a laminating resin is generally not adequate for certain types of structural bonds.
For instance, U.S. Pat. No. 3,321,019 issued May 23, 1967 to G. A. Dimitroff et al for Fiberglass Blade, owned by the assignee of the present application, discloses the formation of a reinforced fiberglass blade for use in helicopters, which blade includes a particular reinforced plastic blade root construction. More specifically, that blade root construction comprises alternate layers of aluminum reinforcing plates and plastic impregnated fiberglass cloth. A suitable primer is provided between each metal plate and layer of fiberglass cloth to prevent metal oxidation and to improve the adhesion between the metal plates and the plastic impregnated fiberglass cloth. Such primers typically take the form of coatings having a thickness less than 1 or 2 mils. The alternate metal and fiberglass sheet laminations are arranged over a spar member and are then adhesively bonded to one another by a vacuum injection process in which liquid plastic impregnates the fiberglass cloth and fills a mold. The resulting structure provides sufficient strength and integrity for its intended function, that being the resistance of shear stresses imposed by bolts which couple the helicopter blade to a central hub. However, the bond between the alternate metal plate and fiberglass laminations may not provide the desired degree of strength and integrity for other applications.
For example, fixed wing aircraft have employed FRP propeller blades for nearly 20 years. These blades have generally comprised a pre-molded FRP shell securely bonded to a central metal spar, with certain spaces between the shell and spar completely filled with a foamtype material. Adequate structural integrity between the fiberglass shell and the metal spar were obtained using certain adhesives, as for instance a thermosetting, non-volatile, modified epoxy resin such as AF111 manufactured by Minnesota Mining & Manufacturing Company which was placed as a film on the spar prior to the bonding of the fiberglass shell therewith. Such adhesives provide a bonding strength which far exceeds that possible by the primers utilized in the aforementioned U.S. Pat. No. 3,321,019 and are thus able to provide the spar-to-shell bond for structures of this particular type and utilization. It will be appreciated, however, that this long standing practice, first required the formation of a molded FRP shell and then required the secondary bonding operation through which the shell and spar were integrally joined.
Accordingly, it is a principal object of the present invention to provide an improved method for manufacturing various fiber-reinforced airfoils, such as propeller blades. Included in this object is the reduction of manufacturing labor while retaining or improving the structural characteristics and dimensional uniformity or repeatability of the product.